What is understood by a multi-level half-bridge here is an electrical circuit that has a connection of two terminals, between which a bridge voltage is present, via two branches meeting at a central terminal, wherein a central voltage relative to the potential of one of the aforesaid terminals is present at the central terminal and wherein the branches configured symmetrically relative to the central terminal respectively have a predetermined number n of switching elements connected in series. These connection points between each two adjacent switching elements of the first branch are respectively connected via a capacitor with the connection points, symmetrically situated relative to the central terminal, between each two switching elements of the second branch. The switching elements of the multi-level half-bridge can be switched to various switched states by using a control unit.
Such a multi-level half-bridge, which is frequently also referred to as a “flying capacitor multi-level converter” or “flying capacitor multi-level converter/inverter” is known in principle from the prior art, wherein an (external) bridge voltage is applied in this case to the (outer) terminals of the multi-level half-bridge and thus the central terminal is used to tap the central voltage as an output voltage.
Corresponding circuits and methods for operation thereof are known, for example, from the following publications:
“Modified Phase-Shifted PWM Control for Flying Capacitor Multilevel Converters”; Feng, C.; Liang. J.; Agelidis V. G.; IEEE Transactions on Power Electronics, Vol. 22, No. 1, January 2007, pages 178-185 and
“Modelling and Control of a Flying-Capacitor Inverter”; Watkins, S. J.; Zhang, L.; School of Electronic and Electrical Engineering, University of Leeds, UK, EPE 2001, Graz.
The term “flying capacitors” is used inasmuch as the respective potential thereof—depending on the actual switched state of the individual switching elements—is constantly shifting relative to the reference potential present at one terminal of the multi-level half-bridge. It is of advantage in this respect that the voltages to be switched by the individual switching elements can be reduced to a fraction of the total voltage to be switched, whereby the requirements for their design are reduced. It is further known that, for given bridge voltage, and depending on the selected charging level of the capacitors, a plurality of different switched states exists for each central voltage—which can be predetermined at various voltage levels—provided this is not 0 or does not correspond to the bridge voltage, and that these states respectively compose the same central voltage redundantly, albeit as a function of the actual switched state of the switching elements, by drawing on various capacitors of the circuit. Nevertheless, the operation of such a half-bridge—even with a substantially uniform load at the central output of the half-bridge—proves to be extremely complex, since the various switched states of the switching elements have a different influence on the charged states of the individual capacitors and since—in the case of too high or too low charging of one of the capacitors—one or more switching elements or capacitors may be destroyed.
Multi-level half-bridges of the type mentioned in the introduction, inasmuch as they are used at all in practice, serve mainly for switching voltages in the high power range. The complex structure and operation of such a multi-level half-bridge has made their use in the medium and low power ranges seem less attractive, however.
According to the conventional prior art, voltage or current converters in the low and medium power range up to the order of magnitude of several hundred kilowatts power are usually designed in the form of standard step-down converters/buck converters, step-up converters/upward converters/boost converters, inverse converters/buck-boost converters, SEPIC converters, Ćuk circuits Ćuk converters, double inverters, zeta converters, forward converters or flyback converters as two-step converters.
In all aforesaid topologies, the switched voltage is reversed between two voltage levels. Therefore these topologies are also referred to as two-step converter topologies. They are subject to harmonics, all the more so the steeper the switching flanks are, to EMC interferences, conversion losses at parasitic capacitances, remagnetization losses in inductors and ohmic losses at the current-carrying components, which are even further intensified by the harmonics and associated high frequencies (skin effect).
Therefore so-called multi-level converters are sometimes used in power engineering, for voltage or current converters for high powers, which typically lie in the megawatt range. Mostly modular or cascadable multi-level converters or cascaded multi-step inverters with separate DC intermediate circuits are used for this purpose, wherein several modules with their own DC intermediate circuits and charge-storage units, acting as two-point networks on the power side, are connected together in series and as half-bridges. These converters are well suited for high powers and are characterized by modularity, good scalability, reduced harmonics, good EMC properties and high efficiency. The structure and control for them are relatively complex, and so these systems are not suitable for lower power classes for the time being.
Flying capacitor multi-level converters in the form of a multi-level half-bridge of the type already mentioned in the introduction are also known, as are so-called diode-clamped multi-level converters.
As already mentioned for the flying capacitor multi-level converters, these are not of as particularly flexible and modular construction as the other aforesaid modular multi-level converters, and so they are not used in practice in power engineering. Moreover, these converters require relatively complex driving and voltage monitoring at the storage capacitors, and so heretofore they have been used hardly or not at all in practice, especially for low or medium powers.